1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser device.
2. Description of the Related Art
Conventionally semiconductor lasers have been widely used as a light source for optical recording apparatuses, optical communications and pumping solid state lasers. Among the semiconductor lasers, the DFB (distributed feedback) type are provided with cyclic bumps and dips within an optical guide in the semiconductor laser to form a diffraction grating, whereby the wavelength is stabilized using a light feedback effect due to the diffraction grating. Because such a DFB laser oscillates in a stable single mode, no longitudinal mode hopping phenomenon caused with a change in temperature will occur and thus a mode hopping noise which is observed in a general Fabry-Perot semiconductor laser will not be generated. Therefore, the DFB laser is especially excellent as a light source of which a low high-frequency noise level is required. Furthermore, the DFB laser has such excellent features that changes in oscillation wavelength with changes in temperature are small and that the oscillation wavelength can be selected by varying a cycle of the diffraction grating, and accordingly it is suitable for light sources for optical communications or for pumping solid state lasers.
FIGS. 6 is a view showing an example of a conventional semiconductor laser device of DFB laser type. FIG. 6A is a general perspective view and FIG. 6B is a partial perspective view showing a shape of a diffraction grating. A semiconductor laser device of DFB laser type is described in Japanese Unexamined Patent Publication JP-A 60-66484(1985), in which are sequentially formed an n-type(hereinafter, denoted by `n-`) Al.sub.0.40 Ga.sub.0.60 As cladding layer 103, a non-doped Al.sub.0.10 Ga.sub.0.90 As active layer 104, a p-type(hereinafter, denoted by `p-`) Al.sub.0.25 Ga.sub.0.75 As optical guide layer 105, an n-GaAs current blocking layer 106 having a stripe-like window, a p-Al.sub.0.40 Ga.sub.0.60 As cladding layer 107 and p-GaAs contact layer 108 on an n-GaAs substrate 102, and electrodes 101, 109 are respectively formed on the bottom face of the substrate 102 and the top surface of the contact layer 108.
As shown in FIG. 6B, diffraction gratings 112, 113 composed of cyclic bumps and dips are formed in a region 111 which is the bottom of the stripe-like window in the top face of the optical guide layer 105, and on the top surface of the current blocking layer 106, respectively. The cladding layer 107 is formed on the diffraction gratings 112, 113 so as to be embeded in the stripe-like window.
In a conventional semiconductor laser device of DFB laser type as shown in FIGS. 6A and 6B, electric current is injected into the active layer 104 through the stripe-like window of the current blocking layer 106. For this end, also in a bottom region, i.e. a current injection region of the stripe-like window of the optical guide layer 105 is formed a diffraction grating.
In processes for forming the diffraction grating such as etching, however, crystalline surfaces are exposed to the atmosphere, and as a result the substrate surface suffers oxidation, which causes many crystal defects. Therefore, in the structure as shown in FIGS. 6A and 6B, the crystal defects concentrate in the vicinity of right above the active layer 104, which forms a portion of poor crystal property.
In such a semiconductor laser, existing crystal defects trigger a further increasing tendency towards crystal defects during the operation, resulting in remarkable loss of life of the semiconductor laser. Furthermore, an increase in internal loss in the laser oscillator occurs and causes a problem of increase in oscillation threshold current or decrease in efficiency.